The present invention relates to a new technique used to improve baseband periodicity, resulting in improved modulation. The disclosed techniques can be used to enhance all forms of radio transmission and reception. In a radio receiver, the audio output depends entirely on the amount of variation in the carrier wave, not upon the strength of the carrier alone.
Carver (U.S. Pat. No. 3,989,897) teaches:
In real time with the incoming signal, a circuit receives and monitors the content of the incoming audio signal and measures or estimates the degree of correlation of the content of such signal. As used herein, the term degree of correlation refers to the degree of periodicity, as contrasted with randomness, of the signal content. Those signals which exhibit a relatively high degree of periodicity are signals which are predictable and are thus considered to have high degree of correlation. On the other hand signal content which is random, that is nonperiodic, is considered to have low degree of correlation. Pure sinusoidal musical sounds are highly correlated while noise is completely random and thus uncorrelated. PA1 In frequency modulated "FM systems", the occupied bandwidth of the transmitter is a function of both modulating signal bandwidth (or symbol rate in digital FM) and the deviation. The deviation is typically a difficult to control parameter leading to one of several compromise situations. The first compromise situation is the use of precision components to decrease the variability of the deviation. Such precision components are rather expensive which leads to a rather substantial increase in the overall cost of the transmitting device. The second compromise situation is the use of labor intensive factory tuning. The factory tuning of a transmitting unit decreases the variability of the deviation but also adds to the cost as well as decreasing product reliability. Yet a third compromise situation is to use neither precision components nor factory tuning. In this third situation, the deviation tolerance is poor, meaning that the unit with the worst case maximum deviation must have an occupied bandwidth that is still within the assigned bandwidth. In this third situation, the average device has a deviation, and corresponding occupied bandwidth which is dramatically underutilized.
Rottinghaus (U.S. Pat. No. 5,483,202) notes:
The largest permissible variation in the carrier wave therefore produces a stronger signal. Modulation percentage is a measurement of this variation. One Hundred Percent (100%) modulation is the condition in which the carrier amplitude during modulation at times is zero and at other times is increased to twice its unmodulated value. This phenomenon occurs when the peak amplitude of the audio frequency current equals the amplitude of the audio frequency unmodulated carrier.
The proportion between the amplitude of the audio frequency current and amplitude of the carrier is called the degree of modulation and can be measured in percentages. For example, if the peak amplitude of the audio frequency signal is equal to three fourths the amplitude of the carrier signal, there is seventy five percent (75%) modulation. Clearly, by treating the audio frequency current, the degree of modulation can be increased. There are a number of processes cited in the prior art that attempt to increase modulation. These attempts all fit in one of several broad categories. These can be outlined as follows, Compression, Excitation or Equalization.
The Compression method results in a compressed signal which improves modulation at specific frequencies at the expense of faithful reproduction of all other frequencies, as well as a reproduced sound that has a substantially altered status. While a compressed signal can increase average deviation power, it is accomplished at a heavy price due to the altered condition of the information content transferred in the modulation process.
Excitation is the method of interjecting additional power into one specific harmonic component of the waveform. This method exaggerates the desired harmonic structure and amplifies the frequency components associated with the spectral excitation. As in compression, the resultant unmodulated signal's information content has been altered. The use of excitation can also increase average deviation voltage. However, as with compression, there is an undesirable effect on the spectral content of the signal.
Equalization is a method of adding power to or subtracting power from large frequency bands within the signal. Where excitation only adds power to a specific frequency within the frequency band, equalization acts on a large discrete set of frequencies. By adding or subtracting power to the desired frequency band, alterations to the frequency envelope will affect information content as well as average deviation. As with the above methods, dramatic alterations to information content, as well as signal envelope shaping, result. These alterations, while allowing increases in the average deviation model, can and often do distort information content in an undesirable fashion.
By realigning the baseband spectral content in a unique way, and making it more periodic, there is less impedance in the baseband signal. This reduction in impedance translates into an increase in the frequency/power density of the spectral model, as will be seen in the following discussion and drawings. With a higher density in the spectral content of the waveform, the average deviation voltage is higher. As a result, modulation is more consistent, thus allowing greater propagation without the undesirable effects of spectral envelope shaping created by all other previous methods.
By aliening spectral content in a unique way, the present invention creates beatless cycles or relationships. This in turn affects the energy dispersal within the signal. The invention allows a greater amount of energy to be distributed among all spectral components. As a result of the baseband signal's greater spectral density, modulation becomes more consistent. This increase in spectral density also increases the average deviation voltage, therefore increasing the average modulation power.
The effect of the present invention on one type of transmission, namely Frequency Modulation (FM), now will be discussed. The greater the RMS and Peak Amplitude of the baseband signal, the greater the frequency variation, or deviation, of an FM modulated carrier. In typical broadcast FM applications, 15 kHz is the highest required audio frequency to be transmitted, and 75 kHz is the largest frequency deviation allowed. The ratio of the maximum allowable deviation to the highest modulating frequency is therefore 75:15. This is known as the deviation ratio. The ratio of the maximum allowable deviation to a specific modulating frequency is called the modulation index. It should be noted that the modulation index and the deviation ratio are the same only at the maximum frequency deviation with the maximum modulating frequency signal.
The deviation ratio of 5 that is used in standard FM broadcasting will produce eight significant sidebands above and below the center frequency with a 15 kHz modulating tone. (The number of sidebands is determined by Bessel functions.) Such a modulating signal will produce sidebands 120 kHz off each side of the modulating center frequency, and will require a total bandwidth of 240 kHz. Since the fundamental frequency of all musical instruments as well as the human voice is below 5 kHz, it is unlikely that a strong signal will ever be transmitted at as high an audio frequency as 15 kHz. When a 5 kHz tone is transmitted, its modulation index is 75/5 or 15. A modulation index of 15 has 20 significant sidebands and will require the following bandwidth: EQU 2(+,-).times.5(dev Ratio).times.20(Sidebands)=200 kHz,
provided the carrier is deviated a full 75 kHz by a 5 kHz tone. While 5 kHz is a relatively high frequency in the audio spectrum, it is occasionally applied to the transmitter with enough amplitude to produce full deviation. However, on average, in standard voice or music broadcast, most frequencies above 5 kHz are unable to produce full deviation. As a result, there is unused modulation potential in the average baseband program signal. The present invention utilizes this potential by realigning spectral content into a more periodic signal allowing improved average deviation and greater power efficiency.
Clearly there is a direct correlation between audio spectral content and modulation. Any improvement to the overall spectral density or other improvements to the baseband signal such as Signal to Noise Ratio, will implant those same improved characteristics onto the unmodulated carrier in the form of improved Carrier to Noise Ratios or other measurable improvements. These improvements to the baseband signal enhance the performance of the modulated carrier signal. Accordingly, by utilizing the present invention in the broadcast chain, the average modulation efficiency is improved. While these data refer to baseband alternating current signals, the principles supported by the current invention can be applied to any other spectral content.